1. The Field of the Invention
The present invention relates to actuators. More specifically, the invention relates to a linear actuator having a low profile and a long stroke.
2. Technical Background
Actuators provide a very flexible and efficient mechanism for converting fluid power into linear or rotary motion. Power may be introduced into the fluid remotely and readily transferred to the actuator through fluid lines or channels. By transmitting power through the fluid, a uniform and constant mechanical motion can be achieved.
While linear actuators include components which move linearly with respect to each other, combining a linear actuator with various gears, hinges, struts, and the like allows nonlinear movement of objects as well. Linear actuators have a broad range of applications. One common example of linear actuators are hydraulic lifts used on heavy equipment.
Generally, a linear actuator includes a power supply, a fluid, a housing, and a piston. Typically, the fluid is a liquid such as hydraulic fluid or a gas such as air. Different fluids transfer the power from the power supply with different characteristics. Selection of the fluid depends largely on the desired application for the linear actuator.
The power supply may be a pump, motor, mechanical lever, or other mechanism for introducing pressure and force into the fluid. Generally, the power supply pressurizes the fluid by adding fluid from a fluid reservoir to a closed system. Alternatively, the power supply may generate additional fluid or excite fluid currently in a closed system.
The housing and piston are coupled such that pressure applied to fluid within the housing causes the piston to extend or retract. The rate at which the fluid is pressurized generally determines the rate the piston moves. The pressurization rate depends largely on the size and capability of the power supply. While the term “piston” is used to describe the member moved by the fluid, the terms piston, ram, and plunger may be used interchangeably depending on the configuration of a particular linear actuator to describe the member which moves like a piston. Generally the term “piston” refers to a member which moves within a housing, however, where the piston and housing are integrated the actuator may be referred to as a piston type actuator. In the specification, in embodiments which integrate the housing and piston, the term “piston” is used for simplicity and clarity.
Linear actuators may be single-acting or double-acting. In a single-acting actuator, the fluid moves the piston within the housing only in one direction. An external force such as gravity returns the piston to a non-extended position. In a double-acting actuator, the fluid is directed such that the fluid may be used both to extend and retract the piston.
Generally, the housing is secured to a stationary member and the piston contacts a moveable member. The piston may or may not be secured to the moveable member. In a single-acting actuator, fluid pressurized within the housing behind the piston extends the piston linearly from the housing.
Unfortunately, linear actuators have limitations. Specifically, most linear actuators are limited to a stroke length less than the length of the housing. Stroke length (also referred to herein as stroke) is the linear distance the piston travels from a fully retracted position to a fully extended position. To operate, generally the housing is a closed system containing the fluid and piston. The housing is at least as long as the piston. The piston retracts within the housing but does not extend beyond the walls of the housing. Therefore, the maximum piston stroke is shorter than the housing length.
To address the limited stroke, telescoping pistons have been developed which include a plurality of stages. The stages are sized and configured to slide within each other like in a telescope. Telescoping stages use the space within the housing more efficiently and allow a stroke which is longer than the housing.
However, telescoping stages generally include separate pieces connected by seals. The seals allow the stages to slide past each other in response to the fluid pressure. Friction between the stages and seals causes the seals to quickly wear. Worn seals may cause fluid to leak between stages. Thus, linear actuators having seals require periodic maintenance.
In addition, separate telescoping stages increase the assembly and production costs for the telescoping pistons. Each stage and seal is a different diameter. Thus, each stage and seal are produced separately and assembled to make the telescoping piston.
Single-acting telescoping actuators may be used for various applications. For example, a single-acting telescoping actuator works well to raise a dump truck bed for dumping a load. However, the potential for leaks, cost of production and assembly, weight, rate of deployment and size may limit the applications of single-acting telescoping actuators.
For example, to use linear actuators in vehicles in some applications, the linear actuators should be inexpensive, light weight, and reliable. A very compact linear actuator having a long stroke may be required. A long stroke generally means the stroke is longer than the length, or profile, of the actuator housing before the piston deploys. In certain applications, linear actuators may be required to deploy very rapidly.
While reciprocating linear actuators may be preferable in certain applications in a vehicle such as a brake system. Other linear actuators may only require a single reliable deployment of the piston. These types of linear actuators may be referred to as “one-shot” linear actuators. For example, components designed to protect and aid an occupant during an emergency may use “one-shot” linear actuators. Unfortunately, conventional single-acting linear actuators have been unable to meet the size, weight, reliability, rapid deployment, low profile and long stroke, and cost requirements for use in vehicle safety systems.
Accordingly, it would be an advancement in the art to provide a linear actuator which is compact having a minimal profile and a long stroke for use with a variety of vehicle safety systems. It would be a further advancement to provide a linear actuator that will rapidly and reliably deploy. It would be another advancement in the art to provide a linear actuator that is inexpensive to fabricate compared to conventional linear actuators. Additionally, it would be an advancement to provide a linear actuator that is light weight and self-contained having no external reservoir. It would be another advancement in the art to provide a linear actuator which is leak proof and requires no maintenance. The present invention provides these advancements in a novel and useful way.